Cepheids as standard candles: a 1% distance scale for cosmology?

Thanks to their period-luminosity (PL) relation, Cepheids provide one
of the most accurate empirical distance scales, applicable up to at
least 20 Mpc. They are in particular at the base of the calibration of
secondary distance indicators, such as SN Ia.

In the era of precision cosmology, the calibration of the PL relation
at the 1% level is however complicated by the fact that long-period
Cepheids are too distant for direct and accurate trigonometric
parallax measurements (even for GAIA). The most accurate Cepheid
distances are currently based on the classical Baade-Wesselink (BW)
technique, which in turn relies on surface brightness-color relations
and a velocity conversion factor (the projection factor). To bypass
this dependance, we applied a novel technique based on the measurement
of the changes in angular diameter of Cepheids using optical
long-baseline interferometry. I will present the results we obtained
from interferometric observations of a sample of Galactic Cepheids,
with an emphasis on the projection factor employed in BW techniques. I
will also briefly discuss the influence of circumstellar envelopes on
these observations, their potential impact on the distance scale, and
present the special case of the dust-embedded Cepheid RS Puppis.

As they are relatively massive stars, Cepheids are often members of
multiple systems. We discovered that they also host bright
circumstellar envelopes, particularly at infrared wavelengths.
Binarity and envelopes can both affect the apparent brightness of
Cepheids, potentially biasing the calibration of the PL relation. But
these properties also provide us with new tools to measure geometric
distances (through binary orbits), and to better understand the
evolution and complex dynamics of the Cepheid atmospheres. I will
present our recent results and ongoing programs on this front.